Turns out this is normal. OpenZFS is released under a different license that is not compatible with the GPL. As a result, ZFS modules are always considered out of tree/proprietary and will taint the kernel.
It also turns out that having a tainted kernel isn’t always a bad thing or a problem. Essentially using any proprietary software that loads kernel modules (Broadcom or Nvidia for example) means you’ve got a tainted kernel.
So while txg_sync hanging is also coincidentally an OpenZFS process, the ZFS modules tainting the kernel are not causing the hang, and txg_sync hanging is not the cause of the kernel being tainted.